Age, the onset of arthritis, prior injuries, poor blood flow, lax tendons or dodgy cartilage...
... take your pick or admit to being a wimp!
On a more serious note, your knees, wrists/thumbs and neck are usually the first to go on bikers.
Being forced to remain locked into unerganomic positions and subject to either severe buffeting or constant vibrations - is it any wonder they call it a day first.
Unless you have a prior history or trauma to the joint, it's most likely down to poor circulation, caused by the cold, static position and to some extent, vibrations.
Putting something like a sock round your knee will help to keep it warmer, but will cause uneven pressure - not helpful if your circulation's poor to start with.
Try with a piece of tubigrip - or alternatively use some ibuprofin gel on your knees prior to a long ride. When I did horseriding & eventing we used to nickname the stuff 12 year old's knees!
Maybe even a bit of Deep Heat may do the trick.
I'm no medical professional by the way - but with dodgy knees I could be considered perhaps an enthusiastic amateur.
Give us a call back when bits have dropped off, been bolted back on, fallen off again, lost, found, discarded and replaced with an assortment of spare parts from Dr Frankenstien's garage sale.
Then you too could join the dubiously illustrious ranks fo the Wonky Knee Gang...
... get you applications in quick - with Mr Toad finally on the road to victory we should have a spot opening up soon.